Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow between the sections. In order to provide galvanic isolation, no direct conduction path is permitted. Energy or information may still be exchanged between the sections by other means, such as capacitance, induction, or electromagnetic waves, or by optical, acoustic, or mechanical means.
Galvanic isolation is used where two or more electric circuits must communicate, but their grounds may be at different potentials. It is an effective method of breaking ground loops by preventing unwanted current from flowing between two units sharing a ground conductor. Galvanic isolation is also used for safety, preventing accidental current from reaching ground through a person's body.
For years, designers of industrial, medical, and other isolated systems had limited options when implementing safety isolation; the only reasonable choice was the optocoupler. Today, digital isolators offer advantages in performance, size, cost, power efficiency, and integration. Isolation imposes constraints such as delays, power consumption, cost, and size. A digital isolator's goal is to meet safety requirements while minimizing incurred penalties.
Optocouplers, a traditional isolator, incur the greatest penalties, consuming high levels of power and typically limiting data rates to below 1 Mbps. More power efficient and higher speed optocouplers are available but impose a higher cost penalty.
Digital isolators using transformer or capacitive coupling have now been available for a number of years and reduce the penalties associated with optocouplers. They may be packaged in a single integrated circuit package and may use CMOS based circuitry to offer significant cost and power savings while significantly improving data rates. Digital isolators typically use foundry CMOS processes and may be limited to materials commonly used in foundries. Nonstandard materials complicate production, resulting in poor manufacturability and higher costs. Common insulating materials include polymers such as polyimide (PI), which can be spun onto a silicon substrate as a thin film, and silicon dioxide (SiO2). Both have well known insulating properties and have been used in standard semiconductor processing for years.
Polymers have been the basis for many digital isolators, giving them an established history as a high voltage insulator. For example, a transformer may be implemented using patterned metal layers on a silicon substrate with polyimide or SiO2 insulation between the metal layers. Current pulses in a primary coil on one layer create magnetic fields to induce current on a secondary coil on a second layer. Similarly, a capacitor may be implemented with thin SiO2 isolation barrier and use capacitive electric fields to couple across the isolation barrier. These types of devices have typically been limited to about 400 V isolation working voltage.
A printed circuit board (PCB) implementation may provide coil windings designed within a PCB layer. These implantations typically have a large footprint and PCB dielectrics may be degraded with moisture conditions, thus limiting isolation barrier capability. Discrete coil wound transformers are bulky and typically have a high cost. A typical discrete coil wound transformer may provide up to approximately 2.5 kV isolation voltage barrier.
High pressure laminate may be used to provide an insulation barrier between coils of a transformer. A typical laminate based signal transformer may be limited to about 2.5 KV isolation voltage breakdown.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.